As a mandrel mill, which is a piece of equipment for manufacturing a seamless pipe or tube, there have been conventionally used a 2-roll type mandrel mill in which two grooved rolls opposed are disposed in each rolling stand, and between adjacent rolling stands, the rolling directions of the grooved rolls are alternately arranged, being 90° shifted, and also a 3-roll type mandrel mill in which three grooved rolls are disposed in each rolling stand such that the angle formed by the rolling directions of the grooved rolls is 120°, and between adjacent rolling stands, the rolling directions of the grooved rolls are alternately arranged, being 60° shifted. Hereinafter, “pipe or tube” is referred to as “pipe” when deemed appropriate.
Herein, in order to enhance the efficiency of manufacturing a seamless pipe, it is desirable that the elongation ratio of a pipe for a mandrel mill (i.e., the ratio of wall thickness of the pipe on the inlet side of the mandrel mill to that on the outlet side thereof) be enhanced as much as possible. This is because, if the elongation ratio of the pipe is enhanced, a pipe with a longer length can be obtained by drawing and rolling, and thus more product stocks can be obtained in one step of drawing and rolling.
In order to enhance the elongation ratio of the pipe for a mandrel mill, it can be conceived, for example, that the wall thickness rolling reduction rate for each of the rolling stands (especially for the upstream rolling stands) is set at a high value. However, if the wall thickness rolling reduction rate is set at too high a value, there arises a possibility that a through-hole called a pinhole defect occurs in a portion of the pipe that is opposed to the flange of the grooved roll, resulting from such a cause as the flow (metal flow) of the pipe material which is rolled at the groove bottom of the grooved roll being not sufficiently spread to the flange side of the grooved roll. Alternatively, for enhancing the elongation ratio of the pipe, it can be conceived to increase the number of rolling stands without setting the wall thickness rolling reduction rate for each of the rolling stands at too high a value. However, increasing the number of rolling stands will increase the equipment cost in proportion, and also require increased manpower for maintenance, thus a practical number of rolling stands is said to be about 5 to 8. Thus, with the conventional mandrel mill, for example, a mandrel mill provided with five rolling stands, the elongation ratio of the pipe is generally held to under 4. Therefore, a mandrel mill which allows the elongation ratio of the pipe to be further enhanced for increasing the efficiency of manufacturing a seamless pipe has been demanded.